Hard disc drives are used in modern computer systems and computer networks to enable users to quickly access vast amounts of electronically stored data. A typical disc drive houses five to ten magnetic discs which are axially aligned and mounted to a spindle motor for rotation at a constant, high speed. An array of read/write heads are controllably positioned adjacent magnetic recording surfaces of the discs in order to store and retrieve the data from tracks defined on the disc surfaces. The heads fly adjacent the recording surfaces on air bearings established by air currents set up by the rotation of the discs.
Of particular interest are heads of the so-called "magneto-resistive" variety, which utilize magneto-resistive (MR) elements to sense the selective magnetization of the tracks during disc drive data transfer operations. A typical MR element is formed from an alloy of materials so as to have a baseline electrical resistance which varies in the presence of a magnetic field of a selected orientation. By passing a bias current through the MR element, the selective magnetization of a corresponding track can be determined in relation to variations in voltage detected across the MR element.
It is common in present generation disc drive manufacturing processes to individually select read bias current magnitudes for each of the MR heads of a disc drive in order to optimize disc drive performance. For example, test data are typically written and then read in turn a number of times using a range of different read bias current magnitudes. Those read bias current magnitudes providing optimum performance are then stored in memory utilized by the drive so that, when a particular head is selected during subsequent operation, the disc drive applies the appropriate read bias current to the particular head.
MR heads are known to be delicate and must be handled and operated with a certain degree of care so as to prevent inadvertent damage which can degrade the reliability of the heads. Although the application of a relatively larger bias current will generally enhance the sensitivity of an MR head during a read operation by providing a higher signal to noise ratio in a recovered readback signal, it is important to ensure that the maximum power dissipation capability of the head is not exceeded. As will be recognized, because the MR element operates as a (highly sensitive) resistance, the power P dissipated by the MR element will be generally proportional to the resistance R of the MR element multiplied by the square of the bias current I (i.e., P=I.sup.2 R). Accordingly, there is an upper limit on the magnitude of the bias current that can be applied to any given MR head, and the application of too large a bias current, even momentarily, can stress the MR head and adversely affect its operational reliability over time.
Even if an MR head is not overstressed, the readback response characteristics of the MR head can still vary significantly over time due to a number of environmental and operational factors. Such readback response characteristics include nominal electrical resistance, as well as signal amplitude, asymmetry and linearity. Electrical resistance is the baseline direct current (dc) resistance of the read element, which will typically be from about 20 to 90 ohms (.OMEGA.). Signal amplitude is a measure of the voltage range between maximum positive and negative peaks in the readback signal. Asymmetry is a measure of a difference between the absolute magnitudes of maximum positive and negative peaks in the readback signal, and linearity is a measure of higher-order response characteristics of the read element at positive or negative transitions in the readback signal. Each of these readback response characteristics affect the ability of an MR head to transduce data from a track.
Accordingly, there remains a continual need for improvements in the art whereby the reliability of a disc drive can be maintained by ensuring that head performance does not degrade to a level where previously stored data cannot be retrieved from a disc.